The use of linearly polarized light is important to modern telecommunication systems and other fiber-optic applications. The efficiency with which such polarization is maintained can be defined and measured using, for example, a polarimeter or more commonly a polarization extinction ratio (PER) meter.
In fiber optics, a PM optical fiber is an optical fiber with intrinsic birefringence in which the polarization of linearly-polarized light properly launched into the fiber is maintained. Known PM fibers may be of various types, such as Panda™, bow-tie (Tiger) type, varying in the shape of so-called stress rods, or tensioning members (e.g. regions of different glass composition in the fiber cladding). Also known are PM fibers that have an elliptical core shape. The intrinsic birefringence of PM fiber creates two orthogonal polarization axes, referred to as the fast axis and the slow axis. The propagation of linearly polarized light with its polarization properly aligned with these axes (i) is faster if the alignment is with the fast axis and (ii) exhibits minimal cross-coupling (i.e. transfer) of optical power to the other axis. The output PER of such fiber is generally high (typically >30 dB), whether the polarization is aligned with the fast or slow PM axis.
As known in the art, fabricating fused couplers from glass fibers generally involves
(i) bringing into intimate contact two or more such fibers under a twisted or parallel arrangement,
(ii) fusing the aligned region under heat to form a single waist, and (iii) drawing such a waist until the desired coupling characteristics are attained.
Fused couplers were originally made from non-PM glass fibers. Such couplers generally provide polarization-insensitive coupling but generally do not preserve polarization. Fused couplers have also been made from glass PM fibers. While such couplers can preserve polarization to generally provide an output PER of greater than 20 dB, their coupling characteristics are generally highly polarization dependent.
To fabricate a fused coupler offering both polarization preservation and polarization-insensitive coupling generally requires a concatenated fiber approach in which the fused coupling region involves non-PM fiber, and the input and output regions involve PM fiber. A key requirement for this concatenated fiber approach is for the non-PM fiber portions to be sufficiently short to prevent depolarization. This approach comprises first assembling PM to non-PM to PM concatenated fibers, and then using either a twisted or parallel arrangement of the non-PM portions to form the fused coupling region. Polarization is preserved by pre-aligning the axes of the PM fiber portions with the plane of the coupler to be formed. However, such alignment can tend to add to and thus increase any twist or stress present in the non-PM fibre. Such increased twist or stress may cause either (i) separation of the non-PM fibers during formation of the fused coupling region or (ii) induction of circular or linear birefringence which can rotate or otherwise degrade the input linear polarization state. Hence, the concatenated fiber approach is difficult to implement and thus may not address the requisite issues of manufacturability and reliability, nor provide any means to compensate polarization changes inadvertently introduced within the fused coupling region. Accordingly, a new concatenated fiber approach is needed for manufacturing more reliable fused fiber couplers with polarization-insensitive coupling and which preserve polarization to a consistently high PER of about 20 dB or more.